1. Technical Field
Several aspects of the present invention relates to an electrochemical device and, more particularly, an electrochemical device having an electrode layer and/or a solid electrolyte layer formed using binder particles, each of the binder particles including a particle and a binder carried on at least a part of a surface thereof.
2. Related Art
Along with development of portable equipments such as a personal computer and a cellular phone, demand for a small-sized lightweight battery as a power source of the portable equipments shows a drastic increase in recent years.
In particular, it is predicted that a lithium battery realizes a high energy density since lithium has a reduced atomic weight and increased ionization energy. Extensive research has been made in this respect, as a result of which the lithium battery is widely used as a power source of the portable equipments these days.
Expansion of a lithium battery market demands a lithium battery having a higher energy density. In order to comply with such a demand, internal energy of the lithium battery has been made greater by increasing the quantity of an active material contained in the battery.
Concomitant with this trend, a noticeable increase has been made in the quantity of an organic solvent contained in an electrolyte (an electrolytic solution) which is a flammable material filled in the battery. This results in an increased danger of battery firing and, therefore, the problem of battery safety becomes at issue in recent years.
One of highly effective methods for assuring the safety of a lithium battery is to replace the electrolyte containing the organic solvent with a nonflammable solid electrolyte. Among others, use of a lithium-ion-conducting inorganic solid electrolyte makes it possible to develop an all-solid lithium battery that exhibits improved safety. Active research is now being made in this connection.
As an example, S. D. Jones and J. R. Akridge, J. Power Sources, 43-44, 505 (1993) discloses an all-solid thin film lithium secondary battery produced by sequentially forming a cathode thin film, an electrolyte thin film and an anode thin film through the use of a deposition apparatus or a sputtering apparatus. It was reported that the thin film lithium secondary battery exhibits superior charge-discharge cycle properties of several thousand cycles or more.
With this thin film lithium secondary battery, however, it is impossible for a battery device to retain an electrode active material in a large quantity, thereby making it difficult to obtain a high capacity battery. In order to increase the battery capacity, a great quantity of electrode active materials should be contained in an electrode.
For this purpose, electrodes of a bulk type battery are composed of an electrode mixture material containing electrolyte particles and electrode active material particles. This makes it possible to maintain an ion-conducting path and an electron-conducting path in the electrodes, and to obtain a bulk type battery having a high capacity.
The bulk type battery is typically manufactured by compression-molding the entire battery device within a mold of a press machine, taking out the battery device from the mold and placing the battery device into a coin type battery container.
However, in the case of the bulk type battery, particularly, an all-solid lithium secondary battery using a sulfide-based lithium-ion-conducting solid electrolyte (a sulfide-based lithium-ion conductor), it is known that the capacity thereof is reduced by about 7% from its initial capacity when subjected to several cycles of charge-discharge operations at most (see, e.g., DENKI KAGAKU, 66, No. 9 (1998)).
Thus, there is currently a demand for development of a bulk type all-solid lithium secondary battery having improved performance and being capable of preventing reduction of a battery capacity over the lapse of charge-discharge cycles.
In the all-solid secondary battery having such a structure, in order to improve strengths of the electrode layers and the solid electrolyte layer, it is proposed that materials containing a binder composed of an organic polymer are used as constituent materials thereof (see, e.g. JP-A-07-161346).
In the case of the solid electrolyte layer containing such a binder, it can be formed by mixing the electrolyte particles with an organic solvent dissolving the binder therein to obtain an electrolyte paste, applying the electrolyte paste onto a substrate, and then drying the same.
In the case of the electrode layer containing the above binder, it can be formed by adding an organic solvent dissolving the binder therein to the electrode mixture material containing electrode active material particles, electrolyte particles and, if need, conducting particles such as carbon particles to obtain an electrode paste, applying the electrode paste onto a substrate, and then drying the same.
As another method, the solid electrolyte layer or the electrode layer can be formed by removing the organic solvent from the above electrolyte paste or the above electrode paste to obtain a solid matter, crushing the obtained solid matter, and then press-molding the same into a mold.
In this case, since the organic polymer as the binder is dissolved in the organic solvent, the organic polymer exists in the organic solvent in a state that it electrically polarizes. As a result, a degree of polarization (adsorption potential) of the organic polymer affects potentials of the solid electrolyte and the electrode active material.
Specifically, in the case where the solid electrolyte layer is formed using a material containing the organic polymer in a polarized state as the binder, the organic polymer affects ion conductivity of the solid electrolyte, as a result of which the ion conductivity of the solid electrolyte layer has often been reduced. From the same reason, electrical resistivity of the formed electrode layer becomes high.
This is because the organic polymer in a polarized state reacts with the solid electrolyte or the electrode active material, and thereby a state that gateways of ion-conducting channels (paths) thereof are sealed by the organic polymer is generated in the paste.
Therefore, in the case where the solid electrolyte layer or the electrode layer is formed by drying such a paste, the above state is maintained in the formed layer. As a result, in the case of the electrode layer, impedance thereof is increased, whereas in the case of the solid electrolyte layer, the ion conductivity thereof is decreased.
For these reasons, internal resistance of the all-solid secondary battery having such electrode layer and solid electrolyte layer becomes extremely high, and an output current thereof becomes low. Therefore, such an all-solid secondary battery lacks in practicality.
In order to solve such a problem, it may be conceived that the solid electrolyte layer or the electrode layer is formed using a mixture material containing dried organic polymer particles as the binder.
However, if the mixture material is prepared by mixing the organic polymer particles with the electrolyte particles or the electrode active material particles when forming the solid electrolyte layer or the electrode layer, the organic polymer particles are bonded together in the mixture material, as a result of which the electrolyte particles or the electrode active material particles cannot be mixed with the organic polymer particles sufficiently.
Therefore, the formed solid electrolyte layer or the electrode layer cannot have excellent strength and a stable electrochemical property. This produces a problem in that an all-solid secondary battery manufactured using such a solid electrolyte layer and/or such an electrode layer also cannot have excellent strength and a stable charge-discharge property.